Semiconductor nanocrystals present numerous technological opportunities that stem from their tunable optical and electrical properties. Their extraordinary extinction coefficients, fluorescence quantum yields, and stability toward photobleaching have lead many researchers to investigate nanocrystals as fluorescent probes in biology. Among the many challenges facing these endeavors, tailoring nanocrystal surfaces to the desired application by ligand exchange has been persistently problematic. For example, attempts to interface nanocrystals with biological molecules have struggled to synthesize sufficiently luminescent nanocrystals with compact ligand shells that are easily conjugated to biomolecules and stable to aqueous conditions. The importance of ligand exchange to emergent nanocrystal technologies underscores the need for an improved description of nanocrystal surface chemistry in general. By focusing on this subject we hope to build a molecular description of nanocrystals, and to improve models of their coordination chemistry, surface structure and reactivity. Ultimately, these fundamental studies will lead us to integrate these extraordinary chromophores in sensing applications. Current descriptions of semiconductor nanocrystal surfaces do not adequately distinguish between dative ligand interactions (L-type binding) and ligands that balance their charge with nonstoichiometric crystals (X-type binding). To address this problem we propose to study the relationship between nanocrystal stoichiometry and the exchange of both X- and L-type surface ligands using multi-nuclear magnetic resonance spectroscopy. We aim to determine the thermodynamic binding constants of ligands and to study the mechanisms and kinetics of their exchange. The use of magic-sized clusters with a well defined structure will further allow us to obtain details about specific aspects of ligand exchange at crystal edges, faces, and vertices. Building upon these studies, we aim to systematically design anchoring chelators (L,X-type ligands) to bind water solubilizing groups irreversibly to the nanocrystal surface. Using HPLC methods the stability of biotin-conjugated nanocrystals will then be assessed. We will also determine, using traditional UV-Vis and photoluminescnece measurements, how these new ligands effect the optical properties of the nanocrystals under investigation. Clarifying the importance of X-type ligands to nanocrystal surface chemistry and its relationship to stoichiometry can have a dramatic influence on all avenues of nanocrystal research. Furthermore, by investigating X-type ligand exchange we stand to gain powerful methods to precisely tailor nanocrystal surfaces. Our studies on new water solubilization methods will lead to biologically relevant nanocrystals with improved size, stability, and solubility properties. These directions are a promising step for nanoscience, on both a fundamental and a technological level. PUBLIC HEALTH RELEVANCE: Nanotechnology has provided a means by which humans can interface with matter at a scale that was previously unobtainable. Living organisms are built of cells that are typically micrometers across and are filled with machinery on the nanometer scale, and thus, nanotechnology has great potential to interrogate and manipulate biology at the cellular level. This proposal aims to better understand the fundamental properties of nanomaterials that govern their structure and function with the ultimate goal of improving the fabrication of nanocrystals for use as biological probes, cellular labels, and drug delivery agents.